The ad features Caren Teves, whose son, Alex, was killed protecting his fiancee in last summer's theater shooting in Aurora, Colo. In April, Teves told the New York Daily News that Flake had lied to her about supporting background checks after he wrote a letter to her claiming he agreed with her on the issue.

The Arizona senator is considered a potential swing vote if the measure comes before Congress again. On Friday, he defended himself on Facebook by saying he supported the Grassley amendment.

"Mayor Bloomberg can spend millions trying to get me to support his view of background checks," Flake wrote. "That’s his call. But we Arizonans aren’t easily bullied. The legislation that would have done the most to keep guns out of the hands of those who shouldn’t have them was the Grassley Amendment. And that’s the amendment I supported."